Polytetrafluoroethylene (PTFE) has been used as a material for a seal ring for oil sealing which has been used as a sliding member under high temperature and high pressure of oil while making the use of characteristics such as non-tackiness, sliding property and heat resistance.
Many of such seal rings for oil sealing having a sliding surface (after this, simply referred to as “seal ring”) are of endless type, for example, as shown in a diagrammatic cross-sectional view of FIG. 1, and are usually used for such an application as shown in a diagrammatic cross-sectional view of FIG. 2. Namely, the seal ring 1 is a ring-like member which is put between the two members (housing 2 and seal ring set body 3) undergoing relative motion at a rotating portion or a sliding portion and is used to seal oil 4. In FIG. 2, the seal ring 1 is wound over the seal ring set body 3 and forms a sliding surface to the housing 2. During the use, the seal ring set body 3 rotates on the housing 2, and a relatively high pressure is applied to the seal ring from the oil 4. Further when a load is applied to the seal ring by repeated normal and reverse rotations, there is a case where breakage of the seal ring occurs relatively earlier, resulting in leaking of the oil 4.
When the seal ring 1 of endless type is wound on the seal ring set body 3, the seal ring 1 is fitted stretched like a rubber band. Therefore the seal ring 1 needs to have a certain degree of elasticity and is required to recover its original form immediately after the fitting.
In order to endow the PTFE seal ring with elasticity (recoverability of form), there have been proposed the following methods:    (1) a method of shortening a sintering time of PTFE,    (2) a method of rapidly cooling after sintering of PTFE, and the like.
However when importance is attached to suitability for fitting (elasticity and recoverability of form), sliding durability under high pressure becomes insufficient, and when importance is attached to sliding durability under high pressure, suitability for fitting becomes inferior.
Such problems of the seal ring are important problems to be solved particularly in the devices of automobile such as a power steering in which importance is placed on the feeling and the safety during driving.
As a result of various studies to develop a seal ring having well-balanced sliding durability under high pressure and suitability for fitting, the inventors of the present invention have found that an intended effect can be obtained by controlling crystallinity of PTFE of the seal ring so that it is within a specific range and have filed the previous patent application.
In the process of conducting further studies with respect to the prior invention, the inventors have found that when a specific modified PTFE is used, even if the crystallinity is set wide, excellent suitability for fitting is obtained while maintaining necessary sliding durability under high pressure, and thus the inventors have completed the present invention.